The present invention generally relates to a band-pass filter employing parallel resonance circuit elements and more particularly, to circuit constructions and actual constructions of such band-pass filters.
As one example of band-pass filters of this kind, there has been conventionally available a band-pass filter having a circuit construction as shown in FIG. 8, with an actual construction as illustrated in FIGS. 9(A) and 9(B).
More specifically, as shown in the circuit construction of FIG. 8, the known band-pass filter has a plurality of dielectric coaxial resonators 1 (three dielectric coaxial resonators 1 in this example) as parallel connection resonance circuit elements, and a coupling line having coupling capacitors C as capacitance coupling elements, and connected to one end of each of said dielectric coaxial resonators 1.
Meanwhile, in the actual construction as shown in FIGS. 9(A) and 9(B), the known band-pass filter includes the plurality of dielectric coaxial resonators 1 (three resonators in this example), a rectangular flat plate-like dielectric substrate 2 and a case 3 in which said resonators 1 and substrate 2 are accommodated. The dielectric coaxial resonators 1 are arranged within the case 3 a parallel relation and directed in the same direction, with each of said dielectric coaxial resonators 1 having an inserted coupling terminal 4 connected for conduction with an inner conductor (not shown) of each dielectric coaxial resonator 1.
The dielectric substrate 2 is disposed at the side of open ends of said dielectric resonators 1 accommodated in the case 3 in the parallel relation to each other, and has coupling electrodes 5 formed at respective predetermined positions on its surface so as to be spaced from each other as shown. In other words, at respective positions of said dielectric substrate 2, the coupling capacitors C composed of said coupling electrodes 5 are formed. To each of the coupling electrodes 5 formed on the dielectric substrate 2, the other end of the coupling terminal 4 inserted in each of the dielectric coaxial resonators 1 is fixed by soldering.
In the conventional band-pass filter having the construction as described above, for providing an attenuation pole at the side of a frequency region higher than the pass-band frequency region thereof, it is necessary to insert an inductance coupling element L between the dielectric coaxial resonator 1 as the parallel resonance circuit element and the coupling capacitors C as the capacitance coupling elements as shown in FIG. 10. In the above case, for the actual construction, it is a general practice to connect an air-core coil 6 acting as an inductance coupling element L between the coupling terminal 4 connected with the inner conductor (not shown) of the required dielectric coaxial resonator 1 and the corresponding coupling electrode 5 formed on the dielectric substrate 2 as shown in FIGS. 11(A) and 11(B).
However, in the above known arrangement, the length of the band-pass filter in the direction of its depth (indicated by an arrow H in FIG. 11(A)) becomes undesirably longer by the extent of the air-core coil 6 thus inserted, with a consequent increase in the size of the band-pass filter on the whole.